he attainment of reproduction is rightly considered to be
the goal towards which many processes in nature are tending. But what is
meant by success? Is it determined by the actual discharge of the sexual
function? So many and so wonderful are the contrivances which have
slowly been evolved to insure this discharge, that it is scarcely
surprising to find attention focused upon this one aspect of the
problem. Yet a moment's reflection will show that so limited a
definition of the term "success" can only be held to apply to certain
forms of life; for where the young have to be cared for, fostered, and
protected from molestation for periods of varying lengths, the actual
discharge of the sexual function marks but one stage in a process which
can only succeed if all the contributory factors adequately meet the
essential conditions of the continuance of the species.

Securing a territory is then part of a process which has for its goal
the successful rearing of offspring. In this process the functioning of
the primary impulse, the acquirement of a place suitable for breeding
purposes, the advent of a female, the discharge of the sexual function,
the construction of the nest, and the rearing of offspring follow one
another in orderly sequence. But since we know so little of the organic
changes which determine sexual behaviour, and have no means of
ascertaining the nature of the impulse which is first aroused, we can
only deal with the situation from the point at which the internal
organic changes reflect themselves in the behaviour to a degree which is
visible to an external observer. That point is reached when large
numbers of species, forsaking the normal routine of existence to which
they have been accustomed for some months, suddenly adopt a radical
change in their mode of behaviour. How is this change made known to us?
By vast numbers of individuals hurrying from one part of the globe to
another, from one country to another, and even from mid-ocean to the
coasts; by detachments travelling from one district to another; by
isolated individuals deserting this place for that; by all those
movements, in fact, which the term migration, widely applied, is held to
denote. Now the impulse which prompts these travelling hosts must be
similar in kind whether the journey be long or short; and it were
better, one would think, to regard such movements as a whole than to
fix the attention on some one particular journey which fills us wit